Image converter tube



A. ROSE IMAGE CONVERTER TUBE Filed Aug. 31, 1959 .gaga

.pag.1

INVENTOR. ALBERr R055 Wc ATTORNEY.

Patented Aug. 27, 1940 narran stares igTENi' @FFME lll/IAGE CONVERTER TUBE Waffe Application August 31, 1939, Serial No. 292,799

6 Claims.

My invention relates to light sensitive devices and more particularly to electron-optical converter tubes capable of high image amplification.

In a conventional image converter tube the electron currents produced by light on a photocathode are often too Weak to produce suflicient fluorescence on the viewing screen. While it has been proposed to interpose between the photocathode and the fluorescent screen one or 10 a number of mesh-like secondary electron emitters for image multiplication purposes, such arrangements have not been altogether satisfactory because of the diiiiculty of obtaining a satisfactory mesh-like electrode having uniform secondary electron emission over its entire surface and because of the decrease in resolution obtainable with such mesh-like electrodes.

It is an object of my invention to providev an electron-optical image converter tube wherein the intensity of the converted image may be greatly increased. It is another object of my invention to increase the intensity of a converted image without introducing distortional effects such as nonuniformity of the converted image.

It is a further object of my invention to provide a .tube of the type described wherein uniform amplification of the converted image may be obtained, and it is a still further object to provide a tube of the electron multiplier type wherein the intensity of the multiplying effect may be increased beyond that obtainable with the type using perforated secondary electron emitters. In accordance with my invention I provide an image converter tube having imperforate planar secondary electron emitters arranged to receive a focused electron image from a photocathode along one path and to deliver a focused secondary electron image to a further secondary emitter or to a uorescent screen along a different path, the

electrons owing along the two paths being acted upon by the Same focusing and deflection means. These and other objects, features and advantages of my invention will become apparent from the following description taken in connection with the accompanying drawing in which:

Fig. l is a perspective view of a tube made in accordance with my invention and Fig. 2 is a plan view showing a modification of my invention.

Referring to Fig. l, the envelope l, which is preferably cylindrical, is closed at one end by an optically transparent window 2 and encloses a substantially transparent photocathode 3 adjacent the window 2 and a secondary electron emitting electrode 4 opposite the photocathode.

(Cl. Z50-150) The transparent photocathode 3 is preferably placed to one side of the longitudinal axis of the envelope l which preferably passes through the center of the secondary emitting electrode il. On the other side of the longitudinal axis from the 5 photocathode 3 and parallel with the photocathode, I provide an electron receiving target such as the fluorescent target 5 which faces the secondary electron emitter il.

In accordance with my invention I provide 10 means to focus the electrons liberated from the photccathode 3 upon the secondary electron emitter il and to simultaneously focus secondary electrons emitted by the secondary emitter [i upon the target 5, said means comprising as shown 15 in Fig. 1 a magnetic coil 6 which surrounds the envelope l and extends over the space separating the photocathode 3 and target 5 from the secondary emitter il. The coil is positioned to generate a magnetic eld having lines of force 20 substantially perpendicular to the electrode structures 3, and 5, the lines of force extending throughout substantially the entire length of the envelope l. Furthermore, I provide means to direct the electrons from the photocathode 3 upon 25 the secondary emitter t and to simultaneously direct secondary electrons emitted by the emitter li upon the target 5, such as a pair of plates 'i and 8 having a separation at least equal to the width of the photocathode 3 and target 5. 30

In operation an optical image such as represented by the arrow 9 is projected upon the photocathode 3 through the lens system lil to liberate electron streams from various areas of the photocathode 3 in accordance with the light 35 and shade areas of the optical image. The ele-ctron streams emitted from the pliotocathode are focused by the magnetic eld generated by the coil 5 and follow substantially straight paths while under the sole inuence of this field. r)The 40 plates 'l and 8 are energized from a source of constant D. C. potential to generate an electrostatic eld which deilects the electrons from the photocathode 3 so that they impinge upon the se-condary electron emitter l even though it is 01T- 45 set from the photocathcde 3. The secondary electron emitter #t is maintained positive with respect to the photocathode 3 by a battery or other direct current potential source Il but vnegative with respect to the target 5, the target 50 5 being maintained highly positive with respect to the secondary emitter i preferably by the same battery or potential source ll. The plates 'l and 8 may be operated at the desired constant potential by a separate battery or potential source i2,

the electrical midpoint of which is preferably maintained at the same positive potential as the target 5.

Referring again to Fig. l., the electrons liberated from the photocathode 3 follow paths such as represented by the path A B While the electrons are under the sole inuence or the magnetic neld. From points B to C the electrons are de- ,eeted by an amount depending upon the potential difference applied to the two plates 'l and [i and the strength of the magnetic field, this derlei-:tion preferably being equivale: to the disstance that the center of the phctocathcde 3 is offset from the center el the secondary emitter' The electrons leave the electrostatic field between he plates t *l l 'i' an t such as at the point C and travel along substantially straight paths to the secondary electron emitter i such as along the path CD, whereupon secondary electrons ated in quantities whici bear a linear ren from the rThese secondary electrons flow a path such represented by the path D--E, approximately the same path as CD, sin e ov this n the magnetic iield rb-y coil il constrains the electrons to flow along the lines of force of this field. At the point however, the secondary electrons diverge from tie path followed by the photo-electrons from the photocathode because o e electrostatic field between the plates 'i and S and are there--ore directed such as alo the path F F and thence along the path F-Cr to the targ t elect ode Since the saine plates i? and i! deflect electrons from the photocathcde 3 to electrons from the emitter to the target 5, it s d rable to arrange the emitter :i si;

sp-cot to the photocathode 3 and tN'get shown in Figure 1, it' is desL able to ter of the secondary electron al or the emitter fr opposite a point the photccathode and target y.son electron receiving areas oi' the an hope. ode

it will be obvious from the above that since the electrons from the photocathode 3 do not have to pass through the secondary emitter i to reach the target, an imperforate secondary emitting electrode may be used, and I therefore prefer to provide this electrode in the form of a sheet of metal having high secondary electron emitting properties. The metal sheet may, however, be treated to obtain high secondary electron emitting properties. Thus, the metal is of silver, its surface may be oxidized and treated with caesium during the sensitization process of the cathode as is well known in the art.

While l have disclosed the target 5 as or the fluorescent type provided by a semi-transparent film of metal with a uorescent coating, it is obvious that when the target 5 is to be viewed directly, the metal sheet need not be of the semitransparent type.

Referring to Fig. 2 which shows a modication or the structure shown in Fig. 1 and wherein similar parts are similarly referenced, I provide a photocatliode 3, an oppositely disposed but oilset secondary electron emitter i and a second secondary electron emitter 2G which is similarly positioned as the target 5 of the construction shown in Fig. 1. The iinal target electrode 2l is placed at the saine end of the tube as the secondary emitter i but oiset therefrom. All of the electrodes 3, ll, 20 and 2l are preferably parallel to one another, the electrodes 3 and 25 being in the same plane and the electrodes ll and 2D likewise being in the same plane but in the opposite end of the envelope.

The operation of the device shown in Fig. 2 is similar to that of Fig. l except that two stages of secondary electron emission are obtained without the necessity of using secondary electron emitting electrodes of the perforated type. The modication of my invention shown in Fig. 2 requires an envelope l having a somewhat larger diameter than that shown in Fig. 1 for an equivaient size of the photocathode but offers the additional advantage that the converted image appearing on the target 2l is viewed from the 0P- posite end of the tube and the observer is removed from the line of sight between the photocathode 3 and the object which it is desired to observe. The secondary emitter 2li may be maintained at the same potential with respect to the emitter and photccathode 3 as was the target 5 as explained in connection with Fig. 1, and the target 2l may be maintained at a still higher positive potential in order to obtain increased light output on the fluorescent coating of the target.

While I do not wish to be limited to any particular theory to explain the operation of my device, it is believed that the electrons, when subjected to the electrostatic field between the plates n landissndsirnultaneously subjected to the magnetic field generated by the coil 5, are deiiected in planes parallel with the plates. rThe electrons acquire transverse velocity from the plat-es 'l and 8 substantially all of which is lost by the electrons when emerging from between the plates, so that the spaciai relationship of the electrons is not destroyed. The electrons impinging on the secondary emitter Il therefore have substantially the saine relative special relationship as when the electrons were emitted by the photocathode 3, and are received on the secondary emitter over areas corresponding to the areas from which they were emitted. It is thus possible with the use of my device to obtain a greatly amplified flow of electrons wherein the density of elemental areas normal to the ow retains the same relationship irrespective of subsequent secondary electron ampiification. The device is therefore suitable for use as an image converter where it is desired to have the converted image correspond to the original optical image.

My improved tube is particularly adapted to image conversion where it is desired to intensify an optical image by forming an intense electron image and projecting the electron image on a target such as a Fluorescent screen to produce the intensified optical image. It will be appreciated, however, that my tube is also adapted for use as an elect-ron multiplying device. The t image converter tube is therefore dennitive of a tube of the type described and is not limited to a tube which converts an optical image into an intensied image on the target but the term may apply to describe a tube wherein the primary purpose is electron multiplication.

While I have indicated the preferred embodiments of my invention and have indicated the speciic application as directed to image converter tubes having a target electrode ol the uorescent type, it will be apparent that my invention is by no means limited to the purpose indicated, to the exact forms illustrated, or to its use in tubes ncorporating targets of the luminescent type, but that many variations may be made in the particular structure used such as by replacing the uorescent target with target electrodes oi the mosaic type, without departing from the scope of the invention as set forth in the appended claims,

What I claim as new is:

l. An image converter tube including an envelope, a photocathode and a target adjacent one end oi the envelope, a secondary electron emitter adjacent the opposite end of the 'envelope and facing said photocathode and target, means to generate a magnetic field having parallel lines of force intercepting said photocathode, said emitter and said target normal to their surfaces and extending along the axial distance separating said photocathode and target from said secondary electron emitter and a pair of plates wholly removed from the direct paths between said photocathode, said emitter and said target adapted to be maintained at fixed direct current potentials, one on either side of the space separating said photocathode and target from said secondary electron emitter the plane of said plates being normal to the planes of said photocathode, said emitter and .said target.

2. An image converter tube having a photocathode, an oppositely disposed secondary electron emitting electrode facing said photocathode, the center of said secondary electron emitting electrode being displaced from a point directly opposite the center of said photocathode, a target adjacent the pho'tocathode to receive electrons from said secondary electron emitting electrode, magnetic coil means extending over and encircling the space separating said photocathode and said target from said secondary electron emitting electrode to generate a magnetic iield having lines of force substantially perpendicular to the said photocathode, target and secondary electron emitting electrode and a pair of plates, one on either side of the space separating said photocathode and said target from said secondary electron emitting electrode, the planes of said plates being normal to said photocathode, said target and said secondary electron emitting electrode to direct electrons emitted from areas on said photocathode to corresponding areas on said secondary electron emitting electrode and to simultaneously direct secondary electrons emitted from areas on said secondary electron emitting electrode to corresponding areas on said target.

3. An image converter tube having an evacuated envelope, a photocathode and a luminescent target electrode adjacent one end of said envelope, the surface of said photocathode being parallel With the surface of said target, an imperforate secondary electron emitter adjacent the opposite end of the envelope and facing said photocathode and target and in a plane substantially parallel with said photocathode and target, a magnetic neld generating coil extending at least over the space separating said photocathode and target from said secondary electron emitter to generate a magnetic iield having lines of force substantially perpendicular to said photocathode, target and secondary electron emitter and a pair of plates on opposite sides of the space separating the said photocathode and target from said secondary electron emitter to direct electrons from said photocathode to said secondary electron emitter and from said secondary emitter to said target when a potential difference is applied to said plates.

4. An image converter tube having an envelope, a photocathode adjacent one end of said envelope, a secondary electron emitter adjacent the opposite end of said envelope facing said photccathode, the center of said electron emitter being displaced from the center of said photocathode, a luminescent target adjacent said photocathode and facing said secondary electron emitter and displaced therefrom by an amount equal to the displacement of said secondary emitter from said photocathode, a magnetic field generating coil surrounding said envelope and extending over the space separating said photocathode and said luminescent target from said secondary electron emitter to generate a magnetic ield having lines of force substantially normal to the surfaces or" said photocathode, secondary emitter and target and a pair of electrostatic deiection plates to direct electrons liberated from said photocathode to said secondary emitter and to direct secondary electrons from said secondary emitter to said target when a constant direct current potential difference is applied to said pair of plates.

5. An image converted tube having an envelope, a set of separated parallel electrodes including a photocathode adjacent one end of said envelope, a second set of separated electron receiving electrodes adjacent the opposite end of the envelope and facing said rst set of electrodes, said second set of electrodes being positioned in planes substantially parallel to said rst set of electrodes, each of the electrodes of said second set being positioned to receive electrons from an electrode of the rst-mentioned set, means comprising an elongated magnetic coil extending over the space separating said sets of electrodes, the longitudinal axis oi said coil being perpendicular to said set oi' parallel electrodes to generate a magnetic eld having lines of force extending along parallel paths from one set of electrodes to the other set and a pair of plates substantially perpendicular to the plates of said sets of electrodes, the said plates being on opposite sides of the path between said sets of electrodes to direct the electrons progressively from an electrode of one set to an electrode of the other set.

6. An image converter tube including an elongated evacuated envelope, a magnetic coil surrounding said envelope to generate a magnetic field having lines of force parallel with the longitudinal axis of said envelope, a photocathode having an electron emitting area and a target electrode having an electron receiving area disposed side by side in a plane perpendicular to the magnetic lines oi force generated by said coil so as to be traversed by different portions of said magnetic field, a secondary electron emitter having a secondary electron emitting area oppositely disposed from, facing and so located with respect to said photoeathode and said target that the center of the secondary electron emitting area of said emitter intercepts a line perpendicular to the plane of said photocathode and said target, cutting said plane midway between the centers of the electron emitting and electron receiving areas of said photocathode and said target, and a pair 

